Lecture 2
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In-class exercise from last time
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Wireshark Project 1 posted, due in a week
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Lab from a different textbook
Work through the lab and answer questions at the
end
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Outline
Chapter 1 - Foundation
1.1 Applications
1.2 Requirements
1.3 Network Architecture
1.4 Implementing Network Software
1.5 Performance
1.6 Summary
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Network Architecture
To deal with the complexity of network design,
engineers have developed network architectures
that are used to design and implement networks.
After discussing general network architecture
concepts we will examine the two most popular
architectures – the OSI and Internet architectures.
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Layering and Protocols
Network architectures
are defined in terms of
abstract layers.
Layering decomposes
the design into simpler
components. It also
allows for a modular
design.
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Fig 1.8:Layered Architecture
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Layering and Protocols
Network architectures
will typically have
multiple abstractions at
any given level.
The abstract objects are
known as protocols.
Fig 1.9: Alternate Abstractions
A protocol provides a communication service that
higher level objects use to exchange messages.
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Layering and Protocols
Fig 1.10:
Service and Peer
Interfaces
A protocol defines two interfaces – a service
interface to higher levels and a peer interface to a
corresponding object on another machine.
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Layering and Protocols
A protocol specification defines the service and
peer interfaces.
The service interface defines the operations the
protocol provides to other local, higher-level
objects. The peer interface defines the form and
meaning of data exchanged with the peer protocol
on another machine.
Ideally a protocol could be implemented in
different ways by different programmers and, if it
adhered to the specification, the implementations
could interoperate.
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Layering and Protocols
Fig 1.11: A Hypothetical
Protocol Graph
RRP: Request/Reply Protocol
MSP: Msg. Streaming Protocol
HHP: Host-to-host Protocol
A suite of protocols can be represented in a
protocol graph – nodes correspond to protocols
and edges represent dependencies.
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Layering and Protocols
Higher-levels
encapsulate data in a
new message (typically
by adding a header) as
data is passed down
the protocol stack.
The headers are
removed as the data
passes up the stack on
the peer.
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Fig 1.12: Message Encapsulation
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Layering and Protocols
Multiplexing and demultiplexing can be achieved
through the use of identifiers (demux keys) in the
header.
For example, different applications would be
assigned different keys by RRP. The key is used
on the peer to direct the message to the correct
corresponding peer application.
Similarly, keys can be used by HHP to demux a
message between RRP and MSP.
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OSI Architecture
The OSI
architecture is
one of the
oldest. It consists
of seven layers.
Refer to the text
for details.
Fig 1.13: The OSI Architecture
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TCP/IP Architecture
The Internet (TCP/IP)
architecture is
overwhelmingly the most
popular architecture in
use. It has made almost
all other architectures
obsolete.
The TCP/IP network
model consists of four
layers.
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Figs 1.14/1.15: The TCP/IP Arch.
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TCP/IP Architecture
TCP/IP can utilize many different Network
protocols (Ethernet, token-ring, serial line, carrier
pigeon). The Network layer is typically
implemented using hardware (a network adapter)
and software (a device driver).
The IP or Internet Protocol defines how messages
are routed between networks. It provides support
for connecting networks to form an internetwork.
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TCP/IP Architecture
The Transport layer (above the IP layer) consists
of two main protocols: TCP (Transport Control
Protocol) and UDP (User Datagram Protocol).
TCP is a reliable, connection-oriented channel
(similar to our theoretical RRP protocol) while
UDP is an unreliable, connection-less channel
(similar to MSP).
The top or Application layer consists of many,
many protocols (FTP, Telnet, SSH, SMTP, HTTP,
etc). Programmers often define their own
protocols for comm. between applications.
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TCP/IP Architecture
The TCP/IP model has three notable features:
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Layering is not strict. Applications can bypass the
Transport layer and use the IP or Network layers.
The model is narrow-waisted. The IP layer can utilize
multiple Network protocols. Many Transport protocols
can be implemented above IP.
The protocol suite can be expanded by adding new
protocols. Official inclusion requires a specification
AND an implementation.
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Implementing Network Software
The success of the Internet can be attributed to
the fact that much of its functionality is provided
by software running on general-purpose
computers.
Knowing how to implement network software is
essential to understanding computer networks.
The way an application uses the network is similar
to the way in which a high-level protocol uses a
lower-level protocol.
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The socket API
The socket interface to the TCP/IP protocol suite
is supported on all popular OSes. Porting
network applications that use the sockets API is
relatively simple.
The socket is the main abstraction in the socket
API. Think of a socket as the point at which an
application connects to the network.
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The socket API
To create a socket in C/C++ call the socket
function:
int socket(int domain, int type, int protocol)
Note: The socket API supports communication
over several network protocol suites (TCP/IP, IPX,
Appletalk, etc) as well as communication between
processes on the same machine (local sockets).
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The socket API
To specify that we want to use the IP protocol,
domain should be AF_INET. (To bypass IP and
use raw packets use AF_PACKET.)
The type argument should be either
SOCK_STREAM to specify TCP or
SOCK_DGRAM to specify UDP.
The protocol argument should be 0 (zero) when
the domain is AF_INET.
An integer identifier (the socket) is returned. It is
used in all subsequent socket functions.
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The socket API - Server Side
If you are writing a server that uses the TCP
transport protocol, you would then call:
int bind(int sck, struct sockaddr *addr, int addrlen)
int listen(int sck, int backlog)
int accept(int sck, struct sockaddr *addr, int *addrlen)
Each of these functions takes a sck argument,
this is the identifier that is returned by a previous
call to socket().
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The socket API - Server Side
A server application waits for clients to connect to
a particular socket address on the local machine.
It does a passive open of the socket
The addr argument used in the bind() call is a
pointer to a data structure that contains the IP
address of the server and a TCP port number.
The TCP port number is used as a demux key to
associate received data with this server.
The addrlen argument is the size of the addr
data structure in bytes.
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The socket API - Server Side
The backlog argument to listen() defines the
maximum length to which the queue of pending
connections for the socket may grow.
The server process will block at accept() until a
client connects.
The addr argument is a pass-back argument, it
contains the IP address and TCP port number of
the client. The addrlen argument is a valuereturn argument. It should be initialized to the
size (in bytes) of the addr structure.
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The socket API - Server Side
The accept() routine returns an index to a new,
active socket. This new socket is used for
communicating with the connected client.
In the event of an error, each of these routines
(socket(), bind(), listen(), accept()) returns the
value -1. You should ALWAYS check for an error
and take appropriate action.
Refer to the man pages for each of these routines
for additional details.
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The socket API - Client Side
After creating a socket via a call to socket(), the
client performs an active open via:
int connect(int sck, struct sockaddr *addr, int addrlen)
The addr argument contains the IP address and
port number of the server the client wishes to
connect to.
On error, -1 is returned. An error may occur due
to network problems or if no server is listening at
the indicated addr.
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The socket API - Communication
After a connection had been established data is
transferred between the server and client using:
int send(int sck, void *buf, size_t len, int flags)
int recv(int sck, void *buf, size_t len, int flags)
recv() will block until a message is available or
the connection is terminated. The write() and
read() routines can used instead of send() and
recv().
Note that sockets are bidirectional.
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In-class Exercises
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After discussion of the simplex server and
client, compile the client and connect to the
server being run at csserver.
Modify the server to display the IP address and
port being used at the client peer
(getpeername()). Run this server on localhost
with a localhost client using hostname
"localhost".
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In-class Exercises
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Use a port number of 0 on the server, so that a
port is dynamically assigned by bind(). Use
getsockname() to determine and display the
port number. Modify the client to accept the
server port number as the second argument.
Run the server on csserver; run the client on
the localhost.
When you have complete this exercise, tar or
zip the client and server files together and email
the archive as an attachment to the instructor at
hwang@evansville.edu
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